The following information is provided to assist the reader to understand the technology described below and certain environments in which such technology can be used. The terms used herein are not intended to be limited to any particular narrow interpretation unless clearly stated otherwise in this document. References set forth herein may facilitate understanding of the technology or the background thereof. The disclosure of all references cited herein are incorporated by reference.
Increasingly, medical devices are being implanted within patients to, for example, treat and/or diagnose various conditions. Implanted medical devices can be used to improve the quality of life as well as to prolong or save lives. Applications for implanted medical devices include, but are not limited to, regulating heart rates, assisting in blood flow, controlling incontinence, helping in hearing, helping to restore control of paralyzed organs and treating depression.
As used herein, the term “powered” refers generally to electrically powered medical devices. As used herein, the term “implanted” refers to a medical device either partially or completely inserted into the body of a patient (for example, a human patient). Often, the implanted medical device is completely or fully inserted into the body. In a number of such devices in which power must be supplied from an external source, power is delivered to the device and/or communications are maintained with the device via percutaneous wires that connect one or more external systems with the implanted device.
Heart assist or blood flow assist devices have been fully implanted within patients to assist the heart in providing adequate blood flow for the needs of the body. Typically, the normal heart provides 1.5 average watts of useful power, which equates to 1.5 joules per second of useful blood work, to satisfy the body's metabolic needs. A severely impaired heart might provide half this power and a heart assist device may make up the difference by providing, for example 0.75 watts of useful power. If the assist device is 15% efficient, it will require a minimum of 5 watts of input power. Heart assist devices in clinical use today use wires that pierce the skin to provide power for the fully implanted assist device. However, use of such percutaneous wires results is a significant risk of infection along the wire track.
In a number of other implanted medical devices in which power must be provided from an external source, a transcutaneous energy transfer system (TETS) is used to wirelessly provide power to the implanted medical device. In a number of such systems, a secondary power coil is implanted and is electrically connected to an implanted rechargeable battery which powers the implanted medical device. A system controller including a primary power coil and a battery is worn by the patient outside of the body. The primary coil transmits energy/power via magnetic force/induction from the external battery across the skin of patient to the secondary coil without requiring piercing of the skin. The external battery can, for example, be removable and rechargeable. Typically, transcutaneous energy transfer systems are used for energy transmission in relatively low power applications (for example, less than 1 watt).
Although it is desirable to develop transcutaneous energy transfer systems for use with implanted heart assist devices to, for example, obviate the risk associated with percutaneous wiring, a number of significant problems persist. As described above, the power requirements for implanted heart assist devices are substantially higher than with many other medical devices, complicating the use of transcutaneous energy transfer systems. Moreover, in the case of continuous flow heart assist devices, loss of power carries a risk of death of up to 40%. A number of precautions, including, for example, use of a plurality of redundant external battery packs, may be required for safety.